1. Field of the Invention
The present invention relates to granite-like artificial stone superior in fabricability, heat resistance, hot water resistance, weather resistance, yellowing resistance, and stain resistance. This artificial stone will find use as a raw material for house furnishings (e.g., bathtub, kitchen counter, and washstand) and interior and exterior decorative walls of buildings.
2. Description of the Prior Art
Artificial stone is well known which is composed of chips or fragments of natural stone embedded in a matrix of unsaturated polyester resin or acrylic resin which is produced by radical polymerization. There is another type of artificial stone which gives the appearance of natural stone on account of the transparent gelcoat (surface layer) covering inside stone chips.
These conventional artificial stones have a disadvantage that the chips of natural stone for decoration do not disperse uniformly but settle easily in the matrix (uncured liquid resin) because of the great difference in specific gravity and that it cannot be given a desired design pattern with transparency and color tone in varied degrees. They also have another disadvantage that the chips of natural stone are liable to interfacial slipping off on account of insufficient adhesion between chips and resin and that they are not necessarily good in strength, durability, and stain resistance. They have an additional disadvantage that they can be fabricated only with the same cutting and grinding tools as used for natural stone and hence their fabrication has to be left to a specialist.
In order to eliminate these disadvantages, there was proposed a new kind of artificial stone in which chips of natural stone are replaced by resin particles obtained by crushing a thermoplastic resin (e.g., acrylic resin) or a cured product of unsaturated polyester resin. (See Japanese Patent Laid-open No. 31134/1984.)
Unfortunately, the artificial stone made with thermoplastic resin particles has a disadvantage that it provides moldings lacking stable quality because the particles are swollen or dissolved by the uncured resin liquid or polymerizable monomer during the molding process and this changes the viscosity and color of the mixture. In addition, the artificial stone of this kind provides moldings which are subject to softening and deforming by heat even at a comparatively low temperature, and scratching and are also liable to discoloration and frosting by cosmetics on account of its poor solvent resistance. Moreover, the artificial stone of this kind is not suitable for bathtubs and washbowls because it whitens and becomes opaque in a short time when it is in contact with hot water.
By contrast, the artificial stone made with particles of cured product of unsaturated polyester resin has improved processability, design freedom, heat resistance, and solvent resistance; yet it is still poor in weather resistance, stain resistance, and heat discoloration (yellowing) resistance, and hot water resistance (resistance to whitening and frosting by hot water).
On the other hand, the above-mentioned artificial stone is based on a matrix resin which is a radical polymerization resin (e.g., an unsaturated polyester resin or an acrylic resin) incorporated with a filler. These matrix resins are poor in heat resistance and weather resistance and are subject to shrinkage at the time of cure. This shrinkage causes cracking and makes demolding difficult in the case of three-dimensional molding.
There has been proposed a method for reducing the shrinkage of matrix resin which takes place at the time of cure. According to this method, the matrix is made from a resin mixture of a radical polymerization resin and a thermoplastic resin, and the latter is subjected to phase separation or foaming at the time of cure. This method, however, has a disadvantage that the resulting cured product is cloudy and opaque (which makes the resulting artificial stone look poor) and is poor in stain resistance.